In the evolution of the mobile cellular standards such as Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA) new modulation techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to be implemented. Introducing cyclic prefix in OFDM makes it robust to delay spread and facilitates very high data rates. OFDM can be regarded as a combination of modulation and multiple-access schemes that segments a communications channel in such a way that many users can share it. Whereas TDMA segments are according to time and CDMA segments are according to spreading codes, OFDM segments are according to frequency. It is a technique that divides the spectrum into a number of equally spaced tones. OFDM then carries a portion of a user's information on each tone. A tone can be thought of as a unique frequency in which each tone is orthogonal with every other tone. FDM typically requires there to be frequency guard bands between the frequencies so that they do not interfere with each other. OFDM allows the spectrum of each tone to overlap, and because they are orthogonal, they do not interfere with each other. By allowing the tones to overlap, the overall amount of spectrum required is reduced. In OFDM, information is modulated onto a tone by adjusting the tone's phase, amplitude, or both. An OFDM system takes a data stream and splits it into N parallel data streams, each at a rate 1/N of the original rate. Each stream is then mapped to a tone at a unique frequency and combined together using the Inverse Fast Fourier Transform (IFFT) to yield the time-domain waveform to be transmitted.
In order to smoothly migrate the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, a new system has to be able to operate on a flexible BW. A proposal for such a new flexible cellular system is Super 3G (S3G), also known as long term evolution of the 3GPP (3GPP LTE), that can be seen as an evolution of the 3G WCDMA standard. S3G will likely use OFDM and will be able to operate on bandwidths (BWs) spanning from 1.25 MHz to 20 MHz. Furthermore, S3G should also be able to work in micro cells, having a radius of about 10 meters, as well as macro cells, having a radius of about 10-100 kilometers. Data rates of up to 100 Mb/s will be possible in the high bandwidth (BW), micro cell system case. In order to achieve those rates, it is anticipated that a different cyclic prefix scheme would be implemented in S3G. There would be one long cyclic prefix used for macro cells with a large delay spread, thereby increasing the overhead and reducing the maximum data rate, and one short cyclic prefix used in small cells, with small multi-path components, thereby decreasing the overhead and increasing the maximum data rate.
The flexibility of the S3G system will introduce new challenges to mobile terminal/user equipment (UE) design. For instance, the variable BW and different cyclic prefix will impose new requirements on synchronization channels for cell search and mobility procedures. In existing cellular systems, such as WCDMA and GSM, a fixed BW is used. A cell search procedure in such system operates as following:
1. For each carrier frequency, receive and down-convert the signal to a baseband signal with BW (equal to the BW of the cellular system (200 kHZ GSM/5 MHz WCDMA) and perform cell search by searching for the cellular system's particular synchronization channels (GSM: FCH, SCH bursts; and WCDMA: P-SCH, S-SCH channels);
2. If a cell is found, correct the carrier frequency (if initial cell search and the mobile terminal/UE local oscillator is not locked to the cellular systems); and
3. Detect the ID of the cell and read the broadcast channel (BCH) and, if the mobile terminal/UE is allowed, camp on the cell (in idle mode) or include the cell in the neighboring set (if active mode).
The search time for the first stage above can be reduced by using a history list (initial cell search) or neighbor list (cell search in active/idle mode) in order to provide a priori knowledge about the used carrier frequencies. The foregoing process is discussed in U.S. patent application Ser. No. 10/315,710, co-owned by Assignee of the Applicant. A conventional synchronization (cell search) procedure for an OFDM system (like WLAN) having fixed Bandwidth and fixed cyclic prefix length is as follows:
1. For each carrier frequency, receive and down-convert the signal to a baseband signal with BW corresponding to the OFDM system BW and slot timing (i.e. SCH channel);
2. Perform coarse frequency correction;
3. Perform fine frequency synchronization (e.g. using the knowledge of the cyclic prefix length);
4. Detect the Cell ID and Read broadcast; and
5. Camp on the cell.
An in-depth discussion of the cell search procedure for a fixed BW OFDM system can be found at Minn, et al., “A Robust Timing and Frequency Synchronization for OFDM Systems”, IEEE Transactions on Communications, Vol. 2 No 4, July 2003 (“Minn”). These conventional cell search solutions can not directly be applied to S3G as S3G has a variable BW and cyclic prefix. What is desired, then, is a fast and accurate cell search procedure for cellular OFDM systems having variable BW.